1. Technical Field of the Invention
The present invention relates to controlling brushless motors, and particularly to a circuit and method for detecting a backward spin of a polyphase motor for use in the control thereof.
2. Background of the Invention
A xe2x80x9cbrushlessxe2x80x9d motor includes a permanent magnet rotor and a stator made up of a number of windings that may be connected in a star configuration or completely independent from each other. In the first case there exist a number of external terminals equal to the number of the motor""s phases. In the second case reference is made to motors with independent phases and both terminals of each winding being available externally. These motors are commonly used in hard and floppy disk drives, DVD (digital video disk) drives, tape video recorders, CD players, etc.
In the majority of cases, brushless motors are three-phase and the driving circuit generally comprises integrated circuits whose output stages drive the phase windings or lines. The integrated circuits may either comprise a three-phase, full-wave, half-bridge circuit in the case of motors connected in a star configuration, or three full-bridge circuits in the case of independent phase motors.
By way of simplification, consider the typical case of a three-phase motor driven in a bipolar mode in which, at each instant, two phase windings are powered, while the third phase winding is momentarily unpowered (full-bridge or half-bridge output node in a state of high impedance). The phase windings or lines are switchingly driven according to a cyclical sequence which must be synchronized with the rotor""s instantaneous position. In a bipolar driving mode, the position may be determined by analyzing the back-electromotive-force (bemf) monitored on the winding that is momentarily unpowered, or sensed by Hall-effect sensors (a more expensive approach that is seldom used). Normally, such a bemf monitoring detects the zero crossing of the bemf that has a sinusoidal or in any case periodic waveform, generally referred to as a xe2x80x9czero-crossxe2x80x9d instant.
Indeed, information on the rotor""s instantaneous position is necessary for driving a synchronous motor, such as a brushless motor.
One application of brushless motors is in disk drive systems in which a brushless motor is utilized to spin the disk on which data is stored. One problem encountered in driving and/or controlling a synchronous motor in a disk drive system is the disk initially spinning in a reverse direction relative to an operative forward direction. In particular, it is desirous for the disk to reliably and predictably spin up to an operable spinning speed from an inoperable or standby state, such as a state following the motor being slowed or spun down due to an absence of new requests for memory access. When a sequence of drive signals are applied to the motor windings during a spin up routine, the initial timing of the sequence of applied drive signals may potentially be incorrect relative to the position of the motor""s rotor, thereby generating negative torque and causing the disk to undesirably spin in a direction that is opposite of the direction of spin during normal operation. This condition is further complicated by the fact that oftentimes different motors and power supplies are used in disk drive systems, thereby presenting additional variables to driving the disk motor which may lead to greater uncertainty in avoiding negative torque and backward disk spin.
Based upon the foregoing, there is a need for preventing the rotor of a brushless, synchronous motor from spinning in a direction that is opposite to a direction of spin during normal operation.
The present invention overcomes the shortcomings in prior systems and thereby satisfies a significant need for a circuit and method for detecting backward spin of a synchronous multi-phase motor for use in applications including disk drive systems. According to a preferred embodiment of the present invention, following the detection of a bemf signal from a first phase line crossing a zero reference, the circuit receives a bemf signal from a second phase winding and/or phase line of a polyphase motor and detects and/or determines a polarity of the bemf signal of the second phase line. The circuit generates a control signal that is selectively asserted indicating the polyphase motor is moving in a reverse direction based upon the detected polarity of the bemf signal of the second phase line. The circuit detects/determines the polarity of the back emf signal of the second phase line following each zero crossing of the back emf signal of the first line.
In particular, the circuit includes a timer element which is enabled upon the zero crossing of the back emf signal of the first phase line. Upon the timer element reaching a predetermined value, the circuit detects/determines the polarity of the back emf signal of the second phase line as mentioned above. The lapse of the predetermined period of time follows the completion of a commutation sequence so that noise and/or other phenomena do not adversely effect the polarity detection of the second phase line.
The circuit preferably includes circuitry for sequentially detecting a polarity of each phase line prior to a zero crossing thereof, and determining whether the motor is spinning in a reverse direction based upon at least one detected polarity of the phase lines.
The control signal generated by the circuit is provided to a processing unit that controls the application of drive signals to the phase lines of the polyphase motor. The control signal may be stored in a flag of a status register for subsequent handling by the processing unit.